1. Field of Invention
The present invention relates to a technology that expands only a part of an encoded image data. Especially it relates to a partial expansion method of an encoded image and an image processing apparatus that decodes (i.e. expands) a part of the encoded image data, such as in JPEG format, and outputs the partially decoded image to a display device according to user control request regarding display area, up/downscaling, scrolling or the like.
2. Description of the Related Art
These days, large volume image data is becoming more common due to increased resolution of digital cameras and mobile phones with camera functionality. However, the resolution of the display device is usually lower than that of the data to be displayed.
For example, a display device to view image data is usually limited up to 0.5 megapixels (VGA)-2 megapixels (HDTV) in resolution. In this case, it is impossible to fully display the entire data of a 10 megapixel image of recent digital cameras. Therefore, as a viewing method, options are “to shrink the image to view its entire image” or “not to shrink the image (or to shrink the image but not enough to fit the display device) and to view a part of the image”. This image processing and image data storage requires significant volume of memory, but because of cost reduction needs and/or hardware restriction, its memory capacity is often restricted.
From these reasons, conventionally, several technologies to reduce memory to be used for image processing or image data storage have been proposed.
For example, Japanese Patent No. 3108283 proposes a partial up/downscaling apparatus that designates start position and end position of up/downscaling according to an RST table storing RST positions which are inserted into encoded JPEG data, reads data from the read starting sector located before the up/downscaling start position to the read ending sector located after the up/downscaling end position out of a storage medium, and up/downscales the data from up/downscaling start position to the up/downscaling end position.
Japanese Patent No. 3399743 proposes a technology that, in order to decode a large encoded image data using small volume of memory with smooth display and scrolling of partially decoded image, a regularly skipped encoded data offset values are recorded from the starting position of encoded image datastream while encoding image data or decoding encoded image data where the encoded data offset values are stored by predetermined intervals, if partial area within source image is selected appropriate encoded data offset values are read out, and encoded image data is partially decoded in the middle of the encoded datastream to perform scrolling.
Also, Japanese Unexamined Patent Application Publication No. 2000-278685 describes a method in detail that partially expands a JPEG image using a scan table to partially expand the source image efficiently.
To create and display a partially decoded image data, certain amount of computational processing time is required. Therefore, Japanese Patent No. 3747914 proposes a technology where a preview image data with reduced resolution is created from a captured source image, and a thumbnail image with further reduced resolution of predetermined size is created, and the source image data are all stored together as an image data set in memory card, and one of the image data in the image data set is selected and displayed from the memory card when in display mode, and if there is no preview image data in the image data set then the source image data or thumbnail image data are displayed on the display device instead.
However, creating control data to partially expand an image and using the control data to partially expand the image require processing costs such as memory space and CPU load. For example, during the creation of control data, when a control request such as displaying an image, scrolling, zoom in, or zoom out is made by a user, responses to the user control request may be delayed due to the control data creation. Even though the speed of the partial decoding is faster than that of the entire decoding, the speed of processes such as scrolling may suffer due to such nature of partial decoding.
With the above circumstances taken into consideration, the object of the present invention is to provide to a partial expansion method of an encoded image and an image processing apparatus to reduce the CPU load of partial expansion processing, and to enable high quality image display while promptly responding to user control requests when displaying the partially decoded image.